Network Working Group A. Melnikov
Internet-Draft Isode Ltd
Intended status: Standards Track D. Cridland
Expires: December 3, 2006 Inventure Systems Ltd
C. Wilson
Nokia
June 2006
IMAP4 Extensions for Quick Mailbox Resynchronization
draft-ietf-lemonade-reconnect-client-01.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
This document defines an IMAP4 extension, which gives an IMAP client
the ability to quickly resynchronize any previously opened mailbox as
part of the SELECT command, without the need for server-side state or
additional client round-trips.
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Changes since draft-ietf-lemonade-reconnect-client-01.txt
o If client's UIDVALIDITY doesn't match server's, the server will
not return any flags anymore.
o Clarified that SELECT (QRESYNC) is a CONDSTORE-enabling command.
o Other minor editorial changes and fixes.
Changes since draft-ietf-lemonade-reconnect-client-00.txt
o Changed server behavior when the specified UIDVALIDITY doesn't
match the current. This allows the client to chose how to proceed
after that.
o Other minor editorial changes and fixes.
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Table of Contents
1. Requirements notation . . . . . . . . . . . . . . . . . . . . 4
2. Introduction and Overview . . . . . . . . . . . . . . . . . . 4
3. Quick resynchronization extension . . . . . . . . . . . . . . 5
3.1. QRESYNC parameter to SELECT/EXAMINE . . . . . . . . . . . 5
4. Formal Syntax . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Normative References . . . . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 8
Intellectual Property and Copyright Statements . . . . . . . . . . 10
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1. Requirements notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively. If a single "C:" or "S:" label applies to
multiple lines, then the line breaks between those lines are for
editorial clarity only and are not part of the actual protocol
exchange.
2. Introduction and Overview
IMAP [CONDSTORE] extension provides a way to quickly resynchronize
changes to IMAP flags since a known moment. This can be done using
FETCH modifier once a mailbox is opened.
The [EXPUNGED] IMAP extension allows to quickly discover expunged
messages.
The IMAP extension described in this document combines functionality
provided by both [CONDSTORE] and [EXPUNGED] extensions, while
reducing one more round-trip.
In particular this extension can be useful for mobile clients that
can experience frequent disconnects caused by environmental factors
(battery life, signal strength, etc.). Such clients would need a way
to quickly reconnect to the IMAP server, without forcing the user to
experience long delay and pay big bills for the amount of traffic
generated by resynchronization.
By extending the SELECT command to perform the additional
resynchronization, this also allows clients to reduce concurrent
connections to the IMAP server held purely for the sake of avoiding
the resynchronization.
[[anchor3: Note to RFC editor: This document is compliant with
"transitional IMAP capabilities" document [TRANS-CAPA]. Please
change the capability name below to "QRESYNC"]]
The quick resync IMAP extension is present if an IMAP4 server returns
"X-DRAFT-W01-QRESYNC" as one of the supported capabilities to the
CAPABILITY command. Note, that this extension REQUIREs support for
the [CONDSTORE] and the [EXPUNGED] IMAP extensions, so they MUST be
announced in the CAPABILITY response as well.
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3. Quick resynchronization extension
3.1. QRESYNC parameter to SELECT/EXAMINE
The Quick Resynchronization parameter to SELECT/EXAMINE commands has
three arguments:
o the last known UIDVALIDITY,
o the last known modification sequence
o and the optional list of known UIDs.
The parameter acts as a CONDSTORE enabling command, as defined in
[CONDSTORE]. In other words, the use of the QRESYNC parameter
implies the CONDSTORE parameter. Also, the QRESYNC parameter implies
the EXPUNGED parameter, as defined in [EXPUNGED].
Before opening the specified mailbox the server verifies all
arguments for syntactic validity. If any parameter is not
syntactically valid, the server return the tagged BAD response, and
the mailbox remains unselected. Once the check is done the server
opens the mailbox as if no SELECT/EXAMINE parameters are specified
(this is subject to processing of other parameters as defined in
other extensions). In particular this means that server MUST send
all untagged responses as specified in Section 6.3.1/6.3.2 of
[RFC3501].
After that the server checks the UIDVALIDITY value provided by the
client. If the provided UIDVALIDITY doesn't match the UIDVALIDITY
for the mailbox being opened, then the server MUST ignore the
remaining parameters and behave as if no dynamic message data
changed. The client can discover this situation by comparing the
UIDVALIDITY value returned by the server. This behaviour allows the
client not to synchronize the mailbox or decide on the best
synchronization strategy.
Example: Attempting to resynchronize INBOX, but the provided
UIDVALIDITY parameter doesn't match the current UIDVALIDITY
value.
C: A02 SELECT INBOX (QRESYNC (67890007 20050715194045000
41,43:211,214:541))
S: * 464 EXISTS
S: * 3 RECENT
S: * OK [UIDVALIDITY 3857529045] UIDVALIDITY
S: * OK [UIDNEXT 550] Predicted next UID
S: * OK [HIGHESTMODSEQ 20060128194045007]
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S: * OK [UNSEEN 12] Message 12 is first unseen
S: * FLAGS (\Answered \Flagged \Draft \Deleted \Seen)
S: * OK [PERMANENTFLAGS (\Answered \Flagged \Draft
\Deleted \Seen \*)] Permanent flags
S: A02 OK [READ-WRITE] Sorry, UIDVALIDITY mismatch
Modification Sequence and UID Parameters:
If the provided UIDVALIDITY matches that of the selected mailbox, the
server then checks the last known modification sequence.
The server sends the client any pending flag changes (using FETCH
responses that MUST contain UIDs) and expunges that have occurred in
this mailbox since the provided modification sequence.
If the list of known UIDs was also provided, the server should only
report flag changes and expunges for the provided messages. If the
client doesn't provide the list of UIDs, the server acts as if the
client has specified "1:*".
Thus, the client client can process just these pending events and
need not perform a full resynchronization. The result of this step
is semantically equivalent to the client issuing:
tag1 UID FETCH "known-uids" (FLAGS) (CHANGEDSINCE
"mod-sequence-value" REPORTEXPUNGES)
Example:
C: A02 SELECT INBOX (QRESYNC (67890007
20060115194045000 41,43:211,214:541))
S: * 314 EXISTS
S: 15 RECENT
S: * OK [UIDVALIDITY 67890007] UIDVALIDITY
S: * OK [UIDNEXT 567] Predicted next UID
S: * OK [HIGHESTMODSEQ 20060115205545359]
S: * OK [UNSEEN 7] There are some unseen messages in the mailbox
S: * FLAGS (\Answered \Flagged \Draft \Deleted \Seen)
S: * OK [PERMANENTFLAGS (\Answered \Flagged \Draft
\Deleted \Seen \*)] Permanent flags
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S: * 49 FETCH (UID 117 FLAGS (\Seen \Answered))
S: * 50 FETCH (UID 119 FLAGS (\Draft $MDNSent))
S: ...
S: * 100 FETCH (UID 541 FLAGS (\Seen $Forwarded))
S: * EXPUNGED 41,43:116,118,120:211,214:540
S: A02 OK [READ-WRITE] mailbox selected
4. Formal Syntax
The following syntax specification uses the Augmented Backus-Naur
Form (ABNF) notation as specified in [ABNF].
Non-terminals referenced but not defined below are as defined by
[RFC3501], [CONDSTORE], [EXPUNGED] or [IMAPABNF].
Except as noted otherwise, all alphabetic characters are case-
insensitive. The use of upper or lower case characters to define
token strings is for editorial clarity only. Implementations MUST
accept these strings in a case-insensitive fashion.
select-param = "QRESYNC" SP "(" uidvalidity SP
mod-sequence-value [SP known-uids] ")"
;; conforms to the generic select-param
;; syntax defined in [IMAPABNF]
uidvalidity = nz-number
known-uids = sequence-set
;; sequence of UIDs, "*" is not allowed
5. Security Considerations
Security considerations relevant to [CONDSTORE] and [EXPUNGED] are
relevant to this extension.
This document doesn't raise any new security concerns not already
raised by [CONDSTORE].
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6. IANA Considerations
IMAP4 capabilities are registered by publishing a standards track or
IESG approved experimental RFC. The registry is currently located
at:
http://www.iana.org/assignments/imap4-capabilities
This document defines the X-DRAFT-W01-QRESYNC [[anchor9: The final
capability name will be chosen during AUTH48]] IMAP capability. IANA
is requested to add this capability to the registry.
7. Normative References
[ABNF] Crocker, D., Ed. and P. Overell, Ed., "Augmented BNF for
Syntax Specifications: ABNF", RFC 4234, October 2005.
[CONDSTORE]
Melnikov, A. and S. Hole, "IMAP Extension for Conditional
STORE Operation or Quick Flag Changes Resynchronization",
RFC 4551, June 2006.
[EXPUNGED]
Melnikov, A., "IMAP4 extension for reporting expunged
messages", June 2006.
[IMAPABNF]
Melnikov, A. and C. Daboo, "Collected Extensions to IMAP4
ABNF", RFC 4466, April 2006.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3501] Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
4rev1", RFC 3501, March 2003.
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Authors' Addresses
Alexey Melnikov
Isode Ltd
5 Castle Business Village
36 Station Road
Hampton, Middlesex TW12 2BX
UK
Email: Alexey.Melnikov@isode.com
Dave Cridland
Inventure Systems Ltd
21, Heol Bronwydd
Caerfyrddin, Cymru SA31 2AJ
GB
Email: dave.cridland@invsys.co.uk
Corby Wilson
Nokia
5 Wayside Rd.
Burlington, MA 01803
USA
Email: corby@computer.org
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